Central Heating Guide
C/H Controls Introduction
C/H Thermal Controls
C/H Valve Controls
C/H Function Controls
C/H System Replacement
C/H Controls Installation
Energy efficient home
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Central Heating Installation - INSTALLING CONTROLS
This section describes the range of central heating system controls and how to install the safely.
TIME SWITCH/PROGRAMMER
Ensure that the unit is suitable, because time
switches can only switch one circuit (eg heating
for combi boilers). Programmers can switch two
circuits (eg heating and hot water). They should
be installed so they can be read and easily reached
and altered. Do not install in positions that
are inconvenient for the householder (eg in
an airing cupboard).
ROOM THERMOSTAT
A room thermostat should be sited in a regularly
heated area that allows free circulation around it. It
should not be exposed to draughts, and should be
away from internal heat sources and direct
sunlight. It should also be in a position that is
readily accessible to the householder, eg not in
cupboards or behind furniture.
The thermostat should be sited about 1.5 m above
floor level, except where the occupants include a
wheelchair user. In this case a suitable height in
excess of 1 m should be agreed with the
householder. The room thermostat should not be
sited where supplementary room heating (eg gas,
electric, solid fuel fire) will affect it. Appropriate
positions will be in the hall or other living room
(ie without supplementary heating). It should not
be sited in a kitchen or combined kitchen/dining
room and should only be sited in a main living
room where it is certain that supplementary
heating is not used.
CYLINDER THERMOSTAT
This usually straps onto the cylinder at a height
of about one-third up from its base – the strap
should be tight for good thermal contact. It should
be adjusted to around 60°C. If set too high it may
give rise to scalding, and if set too low it will
increase risk of the growth of legionella bacteria,
which could lead to serious health problems.
MOTORISED VALVE
Two- and three-port valves are most commonly
used and installation depends on pipework layout
and preference. For example:
three-port valves are suitable for providing
separate heating and hot water circuits; most
three-port valves provide a mid-position so
that shared flow is possible
for more than one heating zone, in addition
to a hot water zone, use a separate two-port
valve for each zone
22 mm valves are usually suitable for boilers
up to around 20 kW; for larger boilers, and
when fitting a motorised valve on a gravity hot
water circuit, 28 mm or larger should be used
the motorised valve is not to be positioned in
the line of the open safety vent pipe or feed
and expansion pipe.
THERMOSTATIC RADIATOR VALVE
They should be installed together with a room
thermostat to provide boiler interlock. Many TRVs
can now be installed in either the flow or return
to the radiator and many modern ones are
bi-directional – if not, it is necessary to ensure that
the water flow is in the correct direction. If more
than half of the radiators in a dwelling are fitted
with TRVs, an automatic bypass will usually be
necessary.
AUTOMATIC BYPASS VALVE
The valve should be installed between the boiler
primary flow and return, noting the direction
of flow. Ensure that the valve has adequate flow
capacity. It should be set correctly so as to ensure
adequate flow rate through the boiler when all
motorised valves and/or TRVs close.
FROST PROTECTION (AIR AND PIPE THERMOSTAT)
Where both air and pipe thermostats are used,
the contacts should be wired in series from a live
supply that is not switched by time switch/
programmer or thermostats, ie protection needs to
be available 24 hours a day. Note that some boilers
already include their own frost protection, but
it should be checked to confirm that the dwelling
is still adequately protected.
WEATHER COMPENSATOR OR UNIT WITH EXTERNAL SENSOR
Where a unit includes an external sensor it is
important to position the sensor on a north-facing
wall, out of direct sunlight and away from other
heat sources.
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